Thousands of years ago, Tsukuyomi realized that Earth was an impure, blood-stained world, which was stealing life from all living beings. So, this sage left Earth and took with him his most trusted relatives, including Eirin Yagokoro, his elder who he relied on the most, and founded the Lunar Capital. The Moon lacked the impurities of Earth and those who took up residence there forsook any lifespan, meaning they were neither alive or dead, but because they are not truly immortal, still stand a faint chance of death of injury.
A few decades ago (presumably around the time of the Apollo project), Lord Tsukuyomi was shocked that the scientific community on Earth has realized the world is built on probabilities. The threat that the beings of Earth and their impurities may reach the Lunar Capital now exists, and the denizens of the Moon have been on alert since then.